The electrical power grid has been around for more than a century, delivering reliable power to customers (rate payers). The costs associated with providing this power may come from two main sources: power generation and delivery. In an ideal situation, power is generated and delivered to customers at the lowest cost possible. Inefficiencies add costs to the overall system and any improvements in increasing efficiency may translate to reduced power generation and delivery costs. One of the causes for power grid inefficiency is the presence of reverse power flow. The problem of reverse power flow is complex as it can be unwelcome as a waste problem or welcome as a surplus of power. Controlling reverse power flow requires a fundamental understanding of the sources that cause reverse power flow, which in turn requires a basic understanding of the structure of the power grid described below.
The power grid is made up of four parts: power generation, transmission grid, distribution grid, and customer load.
Power plants generate power through converting the energy in certain materials (for example, coal, natural gas, petroleum, and nuclear) to electricity. The thermodynamic limits of this conversion process result in approximately two-thirds (˜65%) of energy in the raw materials being converted to electricity. Traditionally, power plants may be located hundreds of miles away from the customers that they serve, which require the establishment of an efficient delivery system between the power plant and the customer. Power lines are responsible for the delivery of power (analogous to water) between the power generators (analogous to the roots of a tree) and the customer (analogous to the leaves on a tree). Power lines can be divided into two main categories: transmission lines and distribution lines.
Transmission lines (analogous to the trunk of a tree) are power lines located between the generator and the substation that make up the transmission grid section of the power grid. Transmission lines are high voltage lines that are used to transfer power over long distances. Traditionally, transmission lines are unidirectional in a point to multi-point topology (analogous to a tree's complex branch structure from one point to multiple points). Power losses in the transmission lines are roughly 2% to 6%.
Distribution lines are power lines located between the substation and the customer sites that make up the distribution grid section of the power grid. Distribution lines may be further divided into two sub-categories: medium voltage lines and low voltage lines. Medium voltage lines (like the branches of a tree) carry power from the substation to the neighborhood community. Medium voltage lines use transformers to reduce the medium voltage lines to low voltage lines (analogous to twigs on the branches), which are the power lines that are familiar to customers (analogous to the leaves on the twigs). Power losses in the distribution lines are roughly 4%.
The customer load can be divided among three types of consumers: industrial, commercial, and residential. The customer load refers to the various amounts of electricity each consumer requires to satisfy their various electrical needs. Roughly 41% to 45% of the power generated from the utility (which shall mean and include all grid participants, including utilities, energy resellers, energy management companies, etc.), is lost before it reaches the customer site. If the power was generated closer to the customer, then the power losses in the transmission lines alone (roughly 6% to 10%) would be an improvement over power provided by the utility. Exacerbating this problem even further, the utility must also overcome a type of power generated at the customer site, which is referred to as reverse power. The direction of this power is from the customer site to the power grid and is called reverse power flow. There are two types of reverse power flow that are described below: reverse power flow from customer loads and reverse power flow from dispersed electrical generators.
Reverse Power-Flow from Customer Loads.
The customer load is made up of all electrical devices within the customer site that use the power provided by the utility. The customer site contributes to the power losses through the electrical characteristics of the customer load. The electrical characteristics of the customer load are made up of all the individual electrical devices combined. This type of reverse power flow is generated by an inefficient customer load. An inefficient customer load may be referred to as a low power factor load. The power associated with this type of reverse power flow creates two problems: customer loads cannot use this power and the utility needs to generate more power because of it. Therefore, this form of reverse power flow is unwelcomed and may be referred to as waste power. Traditionally, distribution lines are bidirectional (allowing power to move in both directions). This bidirectional capability allows for the waste power to re-enter the power grid and is referred to as backfeeding. If waste power is allowed to enter the transmission grid, then it may cause instability of the overall power grid. To prevent waste power from entering into the transmission grid, protection circuits are implemented by the utility to prevent backfeeding at the distribution grid level. A device that corrects an inefficient customer load may be referred to as a power factor correction device. For cost reasons, power factor correction devices are typically not implemented for the entire customer site but are rather implemented on a per product basis. Currently, no actual data is available to determine the amount of energy wasted from inefficient customer loads. If all products were mandated to have an ideal load (power factor of one), then this would equate to the least amount of power that the utility needs to generate which may translate to a lower cost of energy. However, the benefit of a lower cost of energy may not outweigh the additional cost of adding power factor correction in order for a product to have an ideal load. Cost savings can be gained on an individual consumer basis by using energy efficient products but until actual data on wasted power is known, utilities will have a difficult time justifying costs that are focused on correcting inefficient customer loads to rate payers.
Reverse Power-Flow from Dispersed Electrical Generators.
Dispersed electrical generators are small generators connected to the distribution grid. These generators are decentralized, modular, and flexible technologies that are located close to the customer load they serve and are typically less than ten megawatts. Dispersed electrical generators typically use renewable energy sources (for example, hydroelectric, biomass, biogas, solar power, wind power, and geothermal power) and increasingly play an important role for the electric power distribution grid. In the case of residential photovoltaic (PV) systems, the sun is a free source of energy and because the location of the PV system is at the customer site, the losses in power transmission may be negligible. The economic value of using dispersed electrical generators increases as the distance between the utility power plant and the customer increases. Dispersed electrical generators are designed so that the power generated “looks like” utility power and may be considered having near perfect efficiency (the power is synchronized). If the dispersed electrical generator creates more power than the customer load can consume, then the excess power is sent into the distribution grid as a reverse power flow. When dispersed electrical generators create reverse power flow, because the reverse power is synchronized, utilities do not have to produce more power to overcome this type of reverse power flow. The power associated with this type of reverse power flow is welcomed by both customers and the utility. To encourage renewable energy development, utilities have been directed by public utility commissions to adopt policies to buy the excess power produced by customers' dispersed electrical generators. Interconnection agreements in terms of renewable energy contracts are usually made between dispersed electrical generator owners and the utility. For contracts (like net metering contracts), the utility is required to allow the overgeneration of power into the distribution grid. However, utilities find distribution grid instability grows as more dispersed electrical generators are connected. Distribution grid instability occurs when excess power generated by the dispersed electrical generators is sent into the grid. Because of this, net metering contracts are now being replaced with contracts that limit the dispersed electrical generator's generation capacity to less than what is locally consumed.